\chapter*{Preface}

\emph{This is the preface. It will give an idea on how the thesis is structured, the order and origins of the publications, and will give a list of other works done during the doctoral period and not included in the dissertation.}


\emph{One paragraph describing the thesis:}

This dissertation in on \emph{concurrency theory}, the branch of theoretical computer science devoted to the study of theoretical foundations for the analysis and description of concurrent systems. One of the most influential such foundations is represented by \emph{process calculi}. 
These are ``small'' formalisms with a handful of operators intended to capture the essential features of the systems of interest. 
We concentrate on the fundamental properties of process calculi featuring a \emph{higher-order} communication discipline: calculi 
%These are calculi 
in which terms of the terms of the language (i.e. processes) can be passed around.

The main subject of this dissertation is the \emph{expressive power} of higher-order process calculi.
Through expressiveness studies we address also \emph{decidability} issues for these calculi.
Both expressiveness and decidability are issues largely unexplored in the higher-order setting. 
We thus find it convenient to confine ourselves to higher-order process calculi 
in which only process communication is allowed. That is, process calculi in which higher-order communication is \emph{strict}.



\emph{Origin of the chapters:} 
The document is divided in seven chapters, each corresponding to a different expressiveness concern in the higher-order setting. 
Each chapter is intended to be self contained. Part of the content of the thesis has been already published and/or submitted for publication. Chapter X is joint work with NN, as part of my visit to Y, at the end of 2009.

% This document does not include some other research efforts that were carried out during our doctoral studies but that
% fall outside of the main subject of the dissertation.
% Such efforts concern process calculi based on \emph{concurrent constraint programming}, a declarative approach for concurrency with strong ties to logic. They include quantitative extensions for timed variants of such calculi \citep{PerezR08,AP-iclp} and applications to scenarios in session-based, structured communication \citep{LopezOP09}. 
%have appeared in the proceedings of international workshops and conferences